Enabling CoreCLR variation of the test

Value of the KRE_DEFAULT_LIB of the 'k test' process was flowing into Helios resulting in an exception. As the test is always running on desktop clr, the value of this variable always points to the desktop KRE path resulting in the exception. This fix will fix the issue.
I realized that was not required. Since this value is overridden in process space of the `k test` (klr.exe) process before starting iisexpress/klr.exe, the change goes away once the process corresponding to `k test` goes away. Also this change does not have an impact on the `k test` process itself as the process has already started and running smooth. When I rerun the test I can see the original value again during the run of the first variation. Do you still suggest placing the original value back?